Field emission devices using modified carbon nanotubes

ABSTRACT

The present invention relates to a field emission device comprising an anode and a cathode, wherein said cathode includes carbon nanotubes nanotubes which have been subjected to energy, plasma, chemical, or mechanical treatment. The present invention also relates to a field emission cathode comprising carbon nanotubes which have been subject to such treatment. A method for treating the carbon nanotubes and for creating a field emission cathode is also disclosed. A field emission display device containing carbon nanotube which have been subject to such treatment is further disclosed.

This application claims the benefit of U.S. Provisional Application No.60/298,193, filed Jun. 14, 2001, hereby incorporated by reference.

FIELD OF INVENTION

The present invention relates to field emission cathodes which usecarbon nanotubes.

BACKGROUND OF THE INVENTION

Field emission devices are devices that capitalize on the movement ofelectrons. A typical field emission device includes at least a cathode,emitter tips, and an anode spaced from the cathode. A voltage is appliedbetween the cathode and the anode causing electrons to be emitted fromthe emitter tips. The electrons travel in the direction from the cathodeto the anode.

These devices can be used in a variety of applications including, butnot limited to, microwave vacuum tube devices, power amplifiers, ionguns, high energy accelerators, free electron lasers, and electronmicroscopes, and in particular, flat panel displays. Flat panel displayscan be used as replacements for conventional cathode ray tubes. Thus,they have application in television and computer monitors.

Conventional emitter tips are made of metal, such as molybdenum, or asemiconductor such as silicon. The problem with metal emitter tips isthat the control voltage required for emission is relatively high, e.g.,around 100 V. Moreover, these emitter tips lack uniformity resulting innon-uniform current density between pixels.

More recently, carbon materials, have been used as emitter tips. Diamondhas negative or low electron affinity on its hydrogen-terminatedsurfaces. Diamond tips, however, have a tendency for graphitization atincreased emission currents, especially at currents about thirty mA/cm².Carbon nanotubes, also known as carbon fibrils, have been the latestadvancement in emitter tip technology. Although much work has been donein the area of carbon nanotubes as emitter tips in field emittingtechnologies, substantial improvement is still needed, specifically, inthree areas. These areas are reducing work voltage, increasing emissioncurrent, and increasing emission sites.

Reducing the work voltage increases the ease of electron emission andalso increases the longevity of the emitter tips. Increasing both theemission current and the number of emission sites increase thebrightness.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide improved fieldemission cathodes comprising carbon nanotubes as the emitters, whichoperate at reduced working voltage, have increased emissions and moreemission sites.

It is a further object of this invention to provide improved fieldemission cathodes where the emitters comprise treated carbon nanotubes.

It is yet a further object of this invention to provide methods formanufacturing improved field emission cathodes by screen or ink-jetprinting of substrates with inks containing treated or untreated carbonnanotubes.

It is still a further object of this invention to provide improved fieldemission display devices having improved properties such as reducedworking voltage, increased emissions and more emission sites.

SUMMARY OF THE INVENTION

The present invention relates to a field emission cathode comprisingcarbon nanotubes, wherein the nanotubes have been subjected to anenergy, chemical, plasma or mechanical treatment. The carbon nanotubesmay form the cathode or may be deposited onto a substrate to form thecathode.

This invention also relates to a field emission device comprising ananode and a cathode which has been subject to such a treatment.

In one embodiment, the field emission device comprises a substrate, aporous top layer positioned on said substrate, a catalyst materialpositioned on said layer and a cathode positioned on said catalystmaterial, said cathode including a bundle of carbon nanotubes which havebeen subjected to a treatment as described above.

The present invention also includes various field emission displaydevices. In one embodiment, the field emission display device comprisesa first substrate, a first metal film on said first substrate; aconductive polymer film on said first metal film, said conductivepolymer film including emitter tips comprising carbon nanotubes whichhave been subject to a treatment as described above; a dielectric filmon said first metal film; a second metal film on said dielectric film; aspacer; a transparent electrode separated from said second metal film bysaid spacer; a fluorescent material on one side of said transparentelectrode; a second substrate on the other side of said transparentelectrode; and a power supply.

In an alternative embodiment, the field emission display devicecomprises a cathode including carbon nanotubes which have been subjectedto a treatment as described above; an insulating layer on said cathode;a gate electrode on said insulating layer; an anode spaced from saidcathode comprising a phosphor layer, an anode conducting layer and atransparent insulating substrate; and a power supply.

The carbon nanotubes used in the cathodes and field emission devices ofthe invention may be single wall or multi-wall. They comprisesubstantially cylindrical carbon fibrils having one or more graphiticlayers concentric with their cylindrical axes, are substantially free ofpyrolytically deposited carbon overcoat, have a substantially uniformdiameter between 1 nm and 100 nm and have a length to diameter ratiogreater than 5. The carbon nanotubes may be in form of aggregates suchas cotton candy aggregates or bird nest aggregates, as well as in theform of a mat or a film.

Energy treatments may include ion beams, ionizing radiation, atomicbeams, electron beams, ultraviolet light, microwave radiation, gammaray, x-ray, neutron beam, molecular beams and laser beam. Plasmatreatment may be performed with a plasma selected from a groupconsisting of oxygen, hydrogen, ammonia, helium, argon, water, nitrogen,ethylene, carbon tetrafluoride, sulfur hexafluoride, perfluoroethylene,fluoroform, difluoro-dichloromethane, bromo-trifluoromethane,chlorotrifluoromethane and mixtures thereof. Chemical treatment mayinclude acid treatment, metal vapor treatment, chemical vapor transport,and chemical sorption.

The field emission cathode may be formed by dispersing carbon nanotubesinto a liquid vehicle to form a solution; transferring said solution toan electrophoresis bath, said bath including an anode and a cathodeimmersed therein; applying a voltage to said anode and said cathode,thereby causing said carbon nanotubes to deposit onto said cathode;removing said cathode from said bath; heating said cathode; andsubjecting such cathode to a treatment as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, illustrate an exemplary embodiment of thepresent invention.

FIG. 1 is a cross-sectional view of a field emission display deviceusing an modified carbon nanotube cathode according to an exemplaryembodiment of the present invention;

FIG. 2 is a cross-sectional view of a field emission display deviceusing modified carbon nanotubes according to another exemplaryembodiment of the present invention;

FIG. 3 is a cross-sectional view of a field emission display deviceusing modified carbon nanotubes according to another exemplaryembodiment of the present invention;

FIG. 4 is a cross-sectional view of a field emission display deviceusing modified carbon nanotubes according to another exemplaryembodiment of the present invention;

FIG. 5 illustrates an electrophoresis bath used to fabricate a carbonnanotube film (electrode);

FIG. 6 illustrates another electrophoresis bath used to fabricate acarbon nanotube film (electrode);

FIG. 7 illustrates a schematic to measure the differences betweentreated (modified) and untreated field emission characteristics;

FIG. 8 is a plot showing cathode current as a function of voltage formodified carbon nanotubes versus untreated nanotubes in a field emissiondevice;

FIG. 9 is a Fowler-Nordheim plot for modified carbon nanotubes anduntreated nanotubes in a field emission device.

FIG. 10 illustrates a classical field emitter;

FIG. 11 illustrates a field emitting device using ion bombarded carbonnanotubes;

FIG. 12 is a SEM view of the carbon nanotubes on the aluminum substrate.

FIG. 13 illustrates a carbon nanotube mat;

FIG. 14 illustrates the electron emission behavior ofelectrophoretically deposited carbon nanotubes, screen printed carbonnanotubes and carbon nanotube mats in the form of plots of currentdensity as a function of the electric field.

FIG. 15 is a series of photographs of electron emission patterns ofelectrophoretically deposited carbon nanotubes, screen printed carbonnanotubes and carbon nanotube mats.

FIG. 16 is a series of plots of emission characteristics of inkjetprinted carbon nanotubes.

FIG. 17 is a series of photographs of electron emission patterns fromsample inkjet printed carbon nanotubes.

FIG. 18 is a photograph of several inkjet printed carbon nanotubecathodes made from sample 262-67-01.

FIG. 19 is a photograph of several inkjet printed carbon nanotubecathodes made from sample 262-67-02.

FIG. 20 is a photograph of several inkjet printed carbon nanotubecathodes made from sample 262-67-04.

FIG. 21 is a photograph of several inkjet printed carbon nanotubecathodes made from sample 262-68-01.

DETAILED DESCRIPTION OF THE INVENTION

All referenced patents, patent applications, and publications areincorporated by reference herein.

Definitions

“Aggregate” refers to a microscopic particulate structures of nanotubes.

“Assemblage” refers to nanotube structures having relatively orsubstantially uniform physical properties along at least one dimensionalaxis and desirably having relatively or substantially uniform physicalproperties in one or more planes within the assemblage, i.e. havingisotropic physical properties in that plane. The assemblage can compriseuniformly dispersed individual interconnected nanotubes or a mass ofconnected aggregates of nanotubes. In other embodiments, the entireassemblage is relatively or substantially isotropic with respect to oneor more of its physical properties.

“Carbon fibril-based ink” refers to an electroconductive composite inwhich the electroconductive filler is carbon fibrils.

“Graphenic” carbon is a form of carbon whose carbon atoms are eachlinked to three other carbon atoms in an essentially planar layerforming hexagonal fused rings. The layers are platelets having only afew rings in their diameter or ribbons having many rings in their lengthbut only a few rings in their width.

“Graphenic analogue” refers to a structure which is incorporated in agraphenic surface.

“Graphitic” carbon consists of layers which are essentially parallel toone another and no more than 3.6 angstroms apart.

“Nanotube”, “nanofiber” and “fibril” are used interchangeably. Eachrefers to an elongated hollow carbon structure having a diameter lessthan 1 μm. The term “nanotube” also includes “bucky tubes” and graphiticnanofibers in which the graphene planes are oriented in herring bonepattern.

The terms “emitter tips” and “emitters” are interchangeable. The use ofthe word “tip” is not meant to limit the emission of the electrons onlyto the tips of the carbon nanotubes. The electrons can be emitted fromany part of the carbon nanotubes.

Carbon Nanotubes

Carbon nanotubes (CNTs) are vermicular carbon deposits having diametersof less than five hundred nanometers. They exist in a variety of forms,and have been prepared through the catalytic decomposition of variouscarbon-containing gases at metal surfaces, by high temperature carbonarc processes, where solid carbon is used as the carbon feed stock, andby simultaneous laser vaporization of graphite rods and a transitionmetal. Tennent, U.S. Pat. No. 4,663,230, succeeded in growing smalldiameter nanotubes having cylindrical ordered graphite cores and anordered “as grown” graphitic surface uncontaminated with pyrolyticcarbon. Tennent, describes carbon nanotubes that are free of acontinuous thermal carbon overcoat and have multiple graphitic outerlayers that are substantially parallel to the fibril axis. As such theymay be characterized as having their c-axes, the axes which areperpendicular to the tangents of the curved layers of graphite,substantially perpendicular to their cylindrical axes. They generallyhave diameters no greater than 0.1 micron and length to diameter ratiosof at least five. Such nanotubes having graphitic layers that aresubstantially parallel to the fibril axis and diameters between 3.5 and75 nanometers, are described in Tennent et al., U.S. Pat. No. 5,165,909and Tennent et al, U.S. Pat. No. 5,171,560.

The graphitic planes may also be oriented at an angle to the fibrilaxis. Such structures are often called “fishbone” fibrils or nanotubesbecause of the appearance of the two dimensional projection of theplanes. Such morphologies and methods for their production are discussedin U.S. Pat. No. 4,855,091 to Geus, hereby incorporated by reference.

Assemblages and composites consisting of multiwall nanotubes have beendescribed in Tennent et al, U.S. Pat. No. 5,691,054. Such assemblagesand composites are composed of randomly oriented carbon fibrils havingrelatively uniform physical properties. Furthermore, these multiwallnanotubes are substantially free of pyrolytically deposited carbon.

The carbon nanotubes disclosed in U.S. Pat. Nos. 4,663,230, 5,165,909,and 5,171,560, may have diameters that range from about 3.5 nm to 70 nmand lengths greater than 100 times the diameters, an outer region ofmultiple essentially continuous layers of ordered carbon atoms and adistinct inner core region. Simply for illustrative purposes, a typicaldiameter for a carbon fibril may be approximately between about 7 and 25nm, and a typical range of lengths may be 1 μm to 10 μm.

As disclosed in U.S. Pat. No. 5,110,693 and references therein, two ormore individual carbon fibrils may form microscopic aggregates ofentangled fibrils. These aggregates can have dimensions ranging from 5nm to several cm. Simply for illustrative purposes, one type ofmicroscopic aggregate (“cotton candy or CC”) resembles a spindle or rodof entangled fibers with a diameter that may range from 5 nm to 20 μmwith a length that may range from 0.1 μm to 1000 μm. Again forillustrative purposes, another type of microscopic aggregate of fibrils(“birds nest, or BN”) can be roughly spherical with a diameter that mayrange from 0.1 μm to 1000 μm. Larger aggregates of each type (CC and/orBN) or mixtures of each can be formed.

Recently carbon nanotubes having a single wall comprising graphite havebeen produced. These single wall carbon nanotubes have been described inBethune et al., U.S. Pat. No. 5,424,054; Guo, et al., Chem. PhysicsLett., 243:1-12 (1995); Thess, et al, Science, 273:483-487 (1996);Joumet et al., Nature 388 (1997) 756; Vigolo, et al., Science 290 (2000)1331. They are also described in U.S. patent application Ser. No.08/687,665, entitled “Ropes of Single-Walled Carbon Nanotubes” hereinincorporated by reference.

Additional methods of producing single wall nanotubes production havebeen described in PCT Application No. PCT/US99/25702 and PCT ApplicationNo. PCT US98/16071 herein incorporated by reference.

Single wall nanotubes are useful in a variety of applications. Thetubular structure imparts superior strength, low weight, stability,flexibility, thermal conductivity, large surface area and a host ofelectronic properties. They can be used as reinforcements in fiberreinforced composite structures or hybrid composite structures, i.e.,composites containing reinforcements such as continuous fibers inaddition to single wall nanotubes.

The carbon nanotubes may be treated in their as-made form or may bedeposited as a film on a suitable substrate and then treated.

Preparation of Films Containing Carbon Nanotubes

The carbon nanotubes used were obtained from Hyperion CatalysisInternational, Cambridge Mass. They had the designations #1100 and #1100L. Sample #1100 L comprised carbon nanotubes having a so-called BNmacromorphology that had been ball milled in a Red Devil Shaking BallMill for approximately four hours. Some samples were treated with anacid wash of twelve grams of H₃PO₄ in 1.5 liters of water at atmosphericreflux before ball milling. The carbon nanotubes were dried in an ovenbefore ball milling.

The Solution of Nanotubes

The nanotubes were dispersed by known methods in a suitable solvent asis well known in the art, e.g. isopropyl alcohol.

The Substrate

Aluminum substrates were prepared by vapor depositing aluminum ontoglass flats that were approximately 55 mm×45 mm×1 mm in its dimensions.Aluminum adhesion may be enhanced with the addition of an underlyingvapor deposited adhesion layer. A dielectric mask can be applied topattern the aluminum surface into a plurality of electodes prior tonanotube deposition.

The aluminum can also be pretreated to promote the adhesion of thecarbon nanotubes. This can be done with any known pretreatments ofaluminum. The carbon nanotubes can also adhere to other substrates,e.g., SnO₂-in/Sb

The Electrophoresis Bath

The elecrophorectic deposition of the carbon nanotubes was conducted inan electrophoresis bath. The bath consists of a chamber to contain thesolution of carbon nanotubes and means for immersing two opposingelectrodes separated by some distance with the carbon nanotubes betweenthe opposing electrodes. A DC power supply, external to the bath, isused to apply a voltage between the two electrodes immersed in the bath.The cathode lead is connected to the patterned aluminum substrate andthe anode lead is connected to the other electrode. Tantalum was usedfor the second metal. The voltage applied to the two electrodes can beadjusted to a suitable level or the voltage can be adjusted to obtain asuitable current between the two electrodes.

The attachment of carbon nanotubes to the aluminum can be enhanced by abinder. The binders can be a mixture of Ag paste, carbon nanotubes andethanol. Or the binders can be a conductive carbon paste, a conductivemetal paste or a carbonizable polymer.

Electrophoretic Deposition of Carbon Nanotubes on the Substrate

A field emitter substrate is loaded into the electrophoresis bath. Aplurality of cathodes are arranged on a glass substrate, and adielectric film is formed with holes over the cathodes. Metal gates withopenings which are located over the holes of the dielectric film areformed to expose the surface of the cathodes. Then, the carbon nanotubesare uniformly deposited onto the obtained substrate, on the surface ofthe cathodes exposed through the holes by electrophoretic deposition atroom temperature.

Post Deposition Heat Treatment

After the deposition of carbon nanotube particles by electrophoresis,low-temperature heating is performed to sustain the deposition of thecarbon nanotubes on the cathodes and ensure easy removal of impuritieswhich are incorporated into the field emitter during the deposition.

EXAMPLE I Preparation of Nanotube Film on Aluminum Substrate

With reference to FIG. 5, a solution is formed that contains 150 mli-propyl alcohol (IPA) and 0.44 grams of acid washed carbon nanotubes.This solution is placed in an electrophoresis bath 5000.

A patterned, aluminum coated glass substrate 5002 serves as oneelectrode in electrophoresis bath 5000. The pattern forms the pixelsize. The smallest feature size can be ca. 1 micron. The aluminum coatedglass 5002 is about 55 mm×45 mm×1 mm in its dimensions. The aluminumpattern size is about 9 mm×9 mm. The other electrode, tantalum (Ta)electrode 5004 is also inserted into the electrophoresis bath 5000. Aspacer 5006 separates the aluminum coated glass 5002 from the tantalumelectrode 5004. A DC voltage, for example between 40 to 120 volts, e.g.,100 volts is applied to the electrodes. A current between 1.0 to 5 mA,e.g., 3.8 mA, is observed between the electrodes. The duration of thepreparation time can be between about 30 to about 90 minutes, e.g., 60minutes.

FIG. 6 illustrates an alternative electrophoretic method of creating thefilm according to the method disclosed in UK patent application2,353,138 described below. First, a carbon nanotube suspension iscreated. The carbon nanotube particles can have lengths from about 0.1to about 1 μm. The suspension can also include a surfactant, e.g. ananionic, ionic, amphoteric or nonionic, or other surfactant known in theart. Examples of suitable surfactants include octoxynol,bis(1-ethylhexyl)sodium sulfosuccinate, and nitrates of Mg(OH)₂, Al(OH)₃and La(OH)₃.

The suspension is then sonicated to charge the carbon nanotubeparticles. The intensity of the electric field and the time for whichthe electric field is applied define the thickness of the carbonnanotube layer. Greater intensity and longer time yield thicker layers.

With reference to FIG. 6 the field emitter substrate 6030 is loaded intothe electrophoresis bath 6000 containing a carbon nanotube suspension6010. An electrode plate 6020 is also installed in the electrophoresisbath 6000 spaced apart from the field emitter substrate 6030. Thecathode of a DC power supply 6040, which is installed outside of theelectrophoresis bath 6000, is connected to the other cathodes of thefield emitter substrate 6030 and the anode of the DC power supply 6040is connected to the electrode plate 6020. Then, a bias voltage of about1 to about 1000 volts is applied from the DC power supply 6040 betweenthe electrode plate 6020 and the cathodes of the field emitter substrate6030.

As a positive voltage of the DC power supply 6040 is applied to theelectrode plate 6020, carbon nanotube particles charged by positive ionsin the carbon nanotube suspension 6010 migrate to and are attached tothe exposed cathodes of the field emitter substrate 6030, which resultsin the formation of a carbon nanotube film in the pattern of the exposedcathodes.

The height of the printed carbon nanotube film, also known as the ink,coating, or paste, may be less than 10 microns and the space whichisolates carbon nanotube cathodes from the indium tin oxide anode withindium tin oxide and phosphor is about 125 microns.

The electrophoresis process can be applied to both diodes and triodes.For applications to a diode, an electric field having opposite chargesto those on the surfaces of the carbon nanotube particles is applied toexposed electrode surface of a field emitter substrate for selectivedeposition of carbon nanotube particles thereon. For application to atriode having gates, a weak positive electric field is applied to thegates while a positive electric field is applied to the electrodes ofthe field emitter substrate, which avoids deposition of carbon nanotubeparticles on the gates. In particular, the electrode plate is connectedto the anode of the DC power supply and the cathodes of the fieldemitter substrate are connected to the cathode of the DC power supply.As a positive potential is applied to the gates, the gates repelpositive ions in the carbon nanotube suspension at the surface, whilethe exposed cathodes of the field emitter substrate, which are connectedto the cathode of the DC power supply, pull positive ions of thesuspension through the holes. As a result, the carbon nanotubes aredeposited only on the entire exposed surface of the cathodes, not on thegates of the field emitter substrate. At this time, carbon nanotubeparticles are attracted to the field emitter substrate and are orientedsubstantially horizontal, or substantially parallel to the substrate,which allows the carbon nanotube particles to smoothly migrate throughthe holes to the cathodes, and thus the carbon nanotubes can bedeposited.

The film can also be prepared similarly to the carbon ink disclosed inEuropean Patent Application EP 1 020 888 A1—Carbon ink,electron-emitting element, methodfor manufacturing and electron-emittingelement and image display device

Alternative Methods to Prepare Carbon Nanotube Films

In addition to electrophoresis, other processes such as screen printingcan be used for creating the patterns. A screen printing process isdisclosed in U.S. Pat. No. 6,270,369. In addition to screen printing,the carbon nanotubes can be applied to a substrate by ink jet printing.Ink printing is accomplished with carbon nanotube based liquid media orinks in which the fibrils are nearly individualized. Inks typicallycontain a carrier liquid and carbon nanotubes, and may be dried (i.e.evaporate the carrier liquid).

The carbon nanotubes can also be deposited in the form of a mat. Suchporous mats, having densities between 0.10 and 0.40 gm/cc arecoveniently formed by filtration of suspensions of nanotubes asdescribed in U.S. Pat. Nos. 6,099,0965 and 6,031,711. Oxidized nanotubesare easily dispersed in and then filtered from aqueous media. The matsmay be subjected to a rigidization or cross linking step as discussed inthe aforecited patents.

Carbon nanotube mat cathodes have uniform emission sites at relativelylow applied field and may obtain a current density of more than 10mA/cm².

A comparison of the electron emission behavior of electrophoreticallydeposited carbon nanotubes, screen printed carbon nanotubes and carbonnanotube mats in the form of plots of current density as a function ofthe electric field is displayed in FIG. 14. A further comparison of theelectron emission patterns of electrophoretically deposited carbonnanotubes, screen printed carbon nanotubes and carbon nanotube mats isdisplayed in FIG. 15.

Carbon Nanotube Based Inks

In yet another method, fibril based inks can be formulated for use inspray equipment. When combined with a masking technology, spray paintingof fibril based inks offers a suitable method for depositing fibril inkpatterns of either simple or complex designs. Spray painting can also beused to apply a uniform coating over a large area, with or without amasking technology. Spraying equipment can accommodate inks/paints witha wide range of viscosity and thixotropy. Airbrushes are a type ofsprayer widely used in the graphic arts industry and areas where finedetailed spraying is desired.

The inks are sprayed through a stencil (i.e., mask, layer with cut outpattern, etc.) to form the corresponding pattern on the substrate andthe carrier fluid is allowed to evaporate. The ratio of air to ink andthe distance from the substrate can be adjusted to allow the optimumamount of drying of the aerosol droplets before they impinge on thesubstrate surface. In this way the adhesion of the droplets to thesubstrate and the tendency of the ink to run or spread can becontrolled. Once dried the dried ink can have conductivity approachingthat of a bare fibril mat depending on the level of any binder that mayhave been included in the ink formulation.

The compositions are prepared by dispersing oxidized fibrils in waterfirst, then adding other additional ingredients if so desired.

The formation of thin fibril films with these compositions can beachieved by both printing and dip coating. Text and patterns have beenprinted with an Epson® ribbon printer. The surface resistivity ofprinted pattern was measured about 3.5×10⁵ Ω-cm (sample 4 in Table 1).The thickness of the pattern is in the range of few tens of nanometers,corresponding few layers of fibrils. Papers with ˜2.5 mm fibril coatingon both sides have been prepared by dip-coating method. Measured surfaceresistivity for the coated paper is between 200-300 Ω-cm. Bulkresistivity of the fibril coating is ˜5×10⁻² Ω-cm, a number very closeto that measured for a freestanding fibril mat. Furthermore, adhesion offibril films to the paper is excellent due to the strong interactionbetween functional groups on the fibril surface and groups associatedwith cellulose paper. TABLE 1 Composition and Properties of Fibril-BasedInk Composition(%) Resistivity Sample Fibril H₂O EG SS DIOP V(cps) t(μm) ρ_(sur) (Ω-cm) ρ (Ω-cm) 1 2 98 — — — 0 2.5 200-300 5 × 10⁻² 2 4 96— — — 19.2 — — — 3 2.5 77.5 20 — — 0 — — — 4 2.5 77.17 20 0.03 0.3 0 —3.5 × 10⁵ 5 × 10⁻²

Modification of Carbon Nanotube Films

The carbon nanotubes, or film, may be modified by chemical or mechanicaltreatment. The surface may be treated to introduce functional groups.Techniques that may be used include exposing the carbon nanotubes toelectromagnetic radiation, ionizing radiation, plasmas or chemicalreagents such as oxidizing agents, electrophiles, nucleophiles, reducingagents, strong acids, and strong bases and/or combinations thereof. Ofparticular interest is plasma treatment.

Plasma Treatment of Nanotube Films

Plasma treatment is carried out in order to alter the surfacecharacteristics of the carbon fibrils, fibril structures and/or thematrix, which come in contact with the plasma during treatment; by thismeans the fibril composite treated can be functionalized or otherwisealtered as desired. Once equipped with the teaching herein, one ofordinary skill in the art will be able to adapt and utilize well-knownplasma treatment technology to the treatment of such compositematerials. Thus, the treatment can be carried out in a suitable reactionvessel at suitable pressures and other conditions and for suitableduration, to generate the plasma, contact it with the compositematerial, and effect the desired kind and degree of modification.Plasmas such as those based on oxygen, hydrogen, ammonia, helium, orother chemically active or inert gases can be utilized.

Examples of other gases used to generate plasmas include, argon, water,nitrogen, ethylene, carbon tetrafluoride, sulfurhexafluoride,perfluoroethylene, fluoroform, difluoro-dicholoromethane,bromo-trifluoromethane, chlorotrifluoromethane, and the like. Plasmasmay be generated from a single gas or a mixture of two or more gases. Itmay be advantageous to expose a composite material to more than one typeof plasma. It may also be advantageous to expose a composite material toa plasma multiple times in succession; the conditions used to generatethe plasma, the duration of such successive treatments and the durationof time between such successive treatments can also be varied toaccomplish certain alterations in the material. It is also possible totreat the composite material, e.g., coat the material with a substance,wash the surface of the material, etc., between successive treatments.

Plasma treatment of a composite material may effect several changes. Forexample, a composite material comprising a polymer and a plurality ofcarbon fibrils dispersed therein can be exposed to plasma. Exposure toplasma may etch the polymer and expose carbon fibrils at the surface ofthe composite, thus increasing the surface area of exposed carbonfibrils, e.g., so that the surface area of the exposed fibrils isgreater than the geometric surface area of the composite. Exposure toplasma may introduce chemical functional groups on the fibrils or thepolymer.

Treatment can be carried out on individual fibrils as well as on fibrilstructures such as aggregates, mats, hard porous fibril structures, andeven previously functionalized fibrils or fibril structures. Surfacemodification of fibrils can be accomplished by a wide variety ofplasmas, including those based on F₂, O₂, NH₃, He, N₂ and H₂, otherchemically active or inert gases, other combinations of one or morereactive and one or more inert gases or gases capable of plasma-inducedpolymerization such as methane, ethane or acetylene. Moreover, plasmatreatment accomplishes this surface modification in a “dry” process ascompared to conventional “wet” chemical techniques involving solutions,washing, evaporation, etc. For instance, it may be possible to conductplasma treatment on fibrils dispersed in a gaseous environment.

Once equipped with the teachings herein, one of ordinary skill in theart will be able to practice the invention utilizing well-known plasmatechnology. The type of plasma used and length of time plasma iscontacted with fibrils will vary depending upon the result sought. Forinstance, if oxidation of the fibrils' surface is sought, an O₂ plasmawould be used, whereas an ammonia plasma would be employed to introducenitrogen-containing functional groups into fibril surfaces. Once inpossession of the teachings herein, one skilled in the art would be ableto select treatment times to effect the degree ofalteration/functionalization desired.

More specifically, fibrils or fibril structures are plasma treated byplacing the fibrils into a reaction vessel capable of containingplasmas. A plasma can, for instance, be generated by (1) lowering thepressure of the selected gas or gaseous mixture within the vessel to,for instance, 100-500 mTorr, and (2) exposing the low-pressure gas to aradio frequency which causes the plasma to form. Upon generation, theplasma is allowed to remain in contact with the fibrils or fibrilstructures for a predetermined period of time, typically in the range ofapproximately 10 minutes more or less depending on, for instance, samplesize, reactor geometry, reactor power and/or plasma type, resulting infunctionalized or otherwise surface-modified fibrils or fibrilstructures. Surface modifications can include preparation for subsequentfunctionalization.

Treatment of a carbon fibril or carbon fibril structure as indicatedabove results in a product having a modified surface and thus alteredsurface characteristics which are highly advantageous. The modificationscan be a functionalization of the fibril or fibril structure such aschlorination, fluorination, etc., or a modification which makes thesurface material receptive to subsequent functionalization, optionallyby another technique or other chemical or physical modification asdesired.

Chemical Treatments of Nanotube Films

Chemical treatment can be also used. Acid treatments, particularlysevere acids treatments, result in cutting the lengths of nanotubes,sharpening the ends of nanotubes, creating defects on the surface ofnanotubes and introducing functional groups on the surface of nanotubes.Acid treated nanotubes are water dispersible, so chemical treatmentoffers advantages for the formation of nanotube film electrodes.Functional groups can be mostly removed by thermal treatment. Theprocess for doing this is disclosed in U.S. Pat. No. 6,203,814. TheRaman effect or titration can be used to measure the effect of the acidtreatment. Raman can be used to measure the degree of structureimperfection after removing oxygen groups introduced during acidtreatment. The effect of the treatment can be measured by electron spinresonance or simple titration as disclosed therein. See also, R. Khan etal. Electron Delocalization in Amorphous Carbon by Ion Implantation, 63PHYSICAL REVIEW B 121201-1 (2001).

In metal vapor treatment procedures metal atoms can be introduced intonanotube films by heating films under vapor of a metal. For example, Csatoms, which have been shown to enhance field emission, can beintroduced into a nanotube film by placing the film in a vacuum chamberwhich has a Cs source held at 200° C. above (Vapor pressure of Cs at373° C. is 10 mm Hg).

Chemical vapor transport methods can be used. Most metals vaporize atvery high temperature. Metal atoms of these metals, such as Ga, can beintroduced into nanotube films by chemical vapor transport. The nanotubefilm is placed in an evacuated glass tube, at one end. a A metalparticle is placed at the other end. A chemical vapor transport agent,such as Cl₂, I₂, Br₂ and HCl is also included. The tube is placed into athree-zone furnace. The temperature of nanotube is held lower that thatof the metal, so that metal atoms are transported to the nanotube filmsby the transport agent.

Chemical sorption followed by heat treatment can be used. Metal atomscan be introduced into nanotube films by first absorbing a metalcompound like metal halides or organometallic compounds on the surfaceof the nanotube films, followed by heating then under inert gasatmosphere to convert metal halides or organometallic compounds to metalatoms. For example Ge atoms may be introduced into a nanotube film byabsorbing GeBr₂ on the surface of nanotubes from a GeBr2 alcoholsolution, then heating the nanotube film between 200 and 400° C. todecompose GeBr2.

Functionalization of nanotubes by chemical sorption can be used, somemolecules, like metal phthalocyanines may have the effect of lowing workfunction of nanotubes and lead to an enhancement of field emission whenabsorbed on the surface of nanotubes. Absorption can be carried out byplacing a nanotube film electrode in a phthalocyanine, porphyrin ormetalloporphyrin solution; this procedure is disclosed in U.S. Pat. No.6,203,814. The functionalization is carried out by phthalocyanines,metalloporphyrins, porphyrins or other organometallics.

The treatment can also include annealing the film afterfunctionalization. The annealing temperature can be carried out between200 and 900 degrees Celsius in inert gas or under 360 degrees Celsius inair.

Ion Bombardment of Carbon Nanotube Films

The carbon nanotube films, are treated by ion bombardment before use ina field emission device or field emitting cathode.

The settings used to bombard the carbon nanotubes were as follows:

energy: 30 keV. Other ranges appropriate for the present invention canbe from about 5 eV to about 1 MeV, e.g., 10-50 keV.

ion: Ga. Although Ga was used as the ion, any type of ion can be used.Other types of ions, for example, include H, He, Ar, C, O, and Xe.

spot size: defocused, 500 nm. Other ranges appropriate for the presentinvention include from about 1 nm to about 1 micron. Appropriate spotsize can also be based on desired resolution and dose. scan area: 760microns×946 microns Rasterscanned for about twenty seconds. Anyappropriate scan area will suffice.

dose: 2×10¹⁴/cm² ranges include from about 10²/cm² to about 10²⁰/cm²

Additional Methods for Treatments of Nanotube Films

Other energetic beams/sources, including atomic beams, electron beams,neutron beams, molecular beams, lasers, plasmas, UV light, x-ray andgamma rays can be used to treat nanotube films instead of ionbombardment. Mechanical treatment resulting in mechanical disruption,for example, ball milling can be used.

Other characteristics of carbon nanotubes can be modified by the abovetreatments. For example, the treatments can remove surface oxygen,remove insulating oxidation residues, generate edges, points, andsingularities, recrystallize the tubes, generate non-tube carbonnanoparticles. A treatment can also be used to clean the carbonnanotubes, for example cleaning to remove the insulation coatinggenerated by oxidation and cleaning to remove oxygen.

Characterization of the Treated Film

By viewing samples in a SEM it is possible to detect irradiated areas bycontrast change, i.e., dark image. FIG. 12 illustrates scanning electronmicroscope views of carbon nanotubes on aluminum.

FIG. 7 schematically illustrates an apparatus used to make the emissionmeasurements. FIG. 7 illustrates the top view, FIG. 7 a, and side views,FIG. 7 b. FIG. 7 a shows a 6 mm×6 mm phosphor on indium tin oxide (ITO).In FIG. 7 b, the phosphor is shown to be spaced from the patternedcarbon nanotubes by a distance of 125 μm. The entire system is evacuatedwith a vacuum of 5×10⁻⁹ Torr in the emission chamber.

The degree of improvement achieved by ion beam treatment are summarizedin Table 2. TABLE 2 Untreated Treated/Modified CNT Cathode CNT CathodeThreshold voltage 350 Volts 140 Volts Threshold Field 2.8 V/μm 1.1 V/μmEmission current see FIG. 8 6 times increase

The ion bombardment achieves a reduction in work voltage, increasesemission current and increases the number of emission sites. Withreference to FIG. 8, it is seen that the turn-on voltage was reducedsubstantially as the result of ion beam treatment.

FIG. 9 is a Fowler-Nordheim (F-N) plot. The shape of the curves providesthe theoretical proof of FE. Shifting the curve toward the rightside—toward lower voltage—indicates an increased number of emissionsites.

Similar improvements can be obtained by treating the carbon nanotubeswith ultra-violet light, laser beam and plasma.

EXAMPLE II Emission Characteristics of Ion-Beam-Treated Nanotube Films

Carbon nanotube films fabricated by electrophoresis on an aluminum layerdeposited on a glass have been locally irradiated with focused ionbeams. A diode structure with a distance of 125 μm between cathodes andanodes was used for emission measurement. A maximum emission current of375 microamps with a turn-on voltage of 2.8 V/μm for carbon nanotubeemitters was found to decrease by focused ion beam irradiation to 1.1 V/μm with increase in emission current by a factor of six.

The current range that was used in the test was in the low range with ananode voltage of about 400 to 500 volts, close to the turn-on(threshold) voltage for field emission. The change was from 0.05 toabout 0.18 microamps to more than 0.9 microamps with a drastic change inthe F-N plot of FIG. 9.

The physical and chemical effects of ion bombardment on carbon nanotubesare not entirely known. While not wishing to be bound to any particulartheory, it may be that the effect of the ion bombardment is the creationof surface sites which enhance field emission. It is believed that thetreatment 1) cuts lengths of nanotubes, in particular, if high energybeams are used, hence generating more ends; 2) implants ions, like Gaions, into the nanotube film, the ions being inside a single tube andoutside tubes; 3) saturates dangling bonds with hydrogen (where ahydrogen ion beam/plasma is used), resulting in hydrogenated surface; 4)cleans the surface of nanotubes by removing contaminants, such as binderresidue and oxygenated groups; 5) generates localized and delocalizedregions along the nanotube axis by creating pits and carbonnanoparticles and recrystallizing amorphous carbons on the surface ofnanotubes, and disrupting carbon layers, leading to an increasing inemission sites; 6) improves electric contacts between nanotubes.

The surface sites generated by ion bombardment can be defects, which arecarbon atoms at edges, carbon atoms associated with other atoms, like ahydrogen atom, and an implanted Ga atom, and carbon atoms with a sp3configuration or configurations between sp2 and sp3. The defects can beat the ends (exposed) of a nanotube, and on the surface of a singlenanotube associated with a nanoparticle, a pit and a disrupted carbonlayer.

Construction of a Field Emission Display Device Using Treated CarbonNanotube Cathodes

Generally, field emission display devices are based on the emission ofelectrons in a vacuum. Emitter tips emit electrons that are acceleratedin a strong electric field. The electrons ultimately collide withfluorescent materials that emit light. The advantages of this type ofdisplay over other types, such as cathode ray tubes, are that they arevery thin and light and yield high brightness and resolution. Processesfor constructing these devices are disclosed in EP No. 1,073,090 A2.

FIG. 1 shows an exemplary embodiment of a field emission display deviceusing an treated carbon nanotube cathode. The field emission display1000 includes, for example, a first substrate 1010, first metal film1020, a conductive high polymer film 1030, a dielectric film 1040, asecond metal film 1050, a spacer 1060, a transparent electrode 1070, asecond substrate 1080, and emitter tips, the treated carbon nanotubecathode, 1090.

The substrate 1010 is, for example, made of glass quartz, silicon, oralumina (Al₂O₃). Other substrates include silica, platinum, iron and itsalloys, cobalt and its alloys, nickel and its alloys, and ceramics.

The first metal film 1020 functions as the cathode and is, for example,made of chrome, titanium, tungsten, or aluminum. The first metal film1020 has a thickness form about 0.2 to about 0.5 μm.

On the first metal film 1020 is, for example, the dielectric film 1040.The dielectric film 1040 has a thickness from about one to about fiveμm.

On the dielectric film 1040 is the second metal film 1050. The secondmetal film 1050 functions as a gate electrode and is made from, forexample, chrome, titanium, or palladium. The thickness of the secondmetal film is from about 0.2 to 0.5 μm. The second metal film 1050 canalso be patterned, for example, by using a photoresist film that has athickness from about 1.5 to about 2.0 μm. The photoresist film is laterdeveloped forming a photoresist pattern. The accelerating gate electrodeshould be in close proximity to the emitting source approximately one toten μm.

Both the first metal film 1020 and the dielectric film 1040 have aplurality of fine holes. The holes have, for example, a diameter of 0.5to 10.0 μm and are separated from each other by about 2.0 to about 15.0μm.

Formed within the fine holes of the dielectric film 1040 and the secondfilm 1050, is the conductive high polymer film 1030. The conductive highpolymer film 1030 can be, for example, made from carbon adhesive orsilver adhesive. To attach the conductive high polymer film 1030 to thefirst metal film 1020, the conductive high polymer film 1030 isliquefied by heating and poured to fill approximately one-third of eachof the fine holes.

Arranged vertically or horizontally within the conductive high polymerfilm 1030 are carbon nanotubes used as emitter tips 1090. The emittertips 1090 are made from the ion bombarded carbon nanotubes discussedpreviously. These emitter tips 1090 can obtain a great amount ofemission current at a low operating voltage, for example, about 1.5V/μm. The range can be from about 0.1 to about 2.0 V/μm, e.g., about 0.8V/μm to about 1.5V/μm.

Above the second metal film 1050 is the spacer 1060. The spacer 1060 isinstalled to about 100 to about 700 μm. on the second metal film 1050.

The transparent electrode 1070 is on top of the spacer 1060. Thetransparent electrode 1060 functions as an anode and is made of aconducting oxide, such as indium oxide, indium tin oxide, tin oxide,copper oxide, or zinc oxide.

The second substrate 1080 is on the transparent electrode 1070 and canbe made of glass. Fluorescent material, attached to the transparentelectrode 1070, emits red, blue, or green light when electrons contactit.

The emitter tips 1090 are made of the ion bombarded carbon nanotubes.The geometrical features of the emitter tips 1090 should be small. Forexample, the diameters of each emitter tip 1090 should be as small as1.3 nm. The average height of the nanotubes is from about 0.1 to about1000 μm, preferably between 0.1 to about 100 μm. The average diameter isbetween 1.3 to 200 nm depending on whether the nanotubes are singlewalled or multi-walled.

More than 10⁴ emitting tips are needed per pixel of 100×100 μm² assuming50% of nanotube density with a tubule diameter of about 10 to about 100nanometers. The emitter density is preferably at 1/μm², in particularlyat least 10/μm². The entire field emission display 1000 is evacuated.

In FIG. 2, a field emission display 2000 is shown. The field emissiondisplay 2000, includes, for example, a baseplate 2010, a spaced-apartphosphor coated faceplate 2020, and an electron emitter array 2030positioned on the baseplate 2010 for emitting electrons that collidewith the phosphor causing illumination. The components of the fieldemission display 2000 are in a vacuum. The electron emitter array(cathode) 2030 is composed of treated carbon nanotubes that can haveeither an orientation parallel, perpendicular, or any angle between zeroand ninety degrees to the baseplate 2010. (See PCT/US 99/13648—FreeStanding and Aligned Carbon Nanotubes and Synthesis thereof).

FIG. 3 shows yet another embodiment of the field emission device. Thedevice 3000, has, for example, a substrate 3010, a porous top layer3020, a catalyst material 3030, and bundles of treated carbon nanotubes3040 as the cathode.

The substrate 3010 and the porous top layer 3020 can be made of, forexample, silicon. The catalyst material 3030 can be a thin film of ironoxide that is formed in a particular pattern. The carbon nanotubebundles 3040 serve as the cathode. The bundles 3040 are orientedsubstantially perpendicular to the substrate 3010. Alternatively, thebundles 3040 can also be oriented substantially parallel to thesubstrate 3010.

The carbon nanotube bundles 3040 may be about 10-250 μm wide, and up toor greater than three hundred μm in height. The bundles 3040 are of thesame size and shape as the patterns of catalyst material 3030, forexample. The nanotube bundles 3040 can have flat tops or bowl-shapedtops as shown in the figure. The sharp edges of the nanotube bundles3040 function as field emission regions. Each bundle 3040 provides thefield emission for a single pixel in a flat panel display.

The device is evacuated to from about 10⁻³ Torr to about 10⁻⁹ Torr,e.g., from about 10⁻⁷ Torr to about 10⁻⁸ Torr.

The calculation of any electrical field within the device 3000 is madeby taking the applied voltage and dividing it by the distance from theemitter tips to the anode. See (PCT appln. PCT/US99/26332)

FIG. 4 shows another embodiment of a flat panel field emission display4000. The display 4000, for example, includes cathode 4010 that containsa plurality of treated carbon nanotube emitting tips 4020 and an anode4030. The anode 4030 further includes an anode conductor 4040 and aphosphor layer 4050. Between the cathode 4010 and the anode 4030 is aperforated conductive gate electrode 4060. Between the gate electrode4060 and the cathode 4010 is an insulating layer 4070. The space betweenthe anode 4030 and the carbon nanotube emitting tips are sealed andevacuated. The voltage is supplied by a power supply. The electronsemitted from the emitting tips 4020 are accelerated by the gateelectrode 4060, and move toward the anode conductor layer 4080 which isa transparent conductor such as indium-tin oxide. The gate electrode4060 should be within 10 μm of the emitting tips 4020. As the emittedelectrons hit the phosphor layer 4050, light is given off. (see, EP1,022,763 A1). The colors of the emitted light depend on the phosphorsthat are used. For example Zn:Scu, Al for green, Y₂O₃:Eu for Red, andZnS:Ag for blue.

The cathodes and anodes can be referred to as sources and drainsrespectively.

Operation of a Field Emission Device

To operate the field emission display device, the treated carbonnanotube cathode is held at a negative potential relative to the anode.As a result of this potential difference, electrons are emitted from theemitter tips and travel to the anode. The gate electrode can be used toaccelerate the emitted electrons.

Field Emission Display Devices

Using the ion bombarded carbon nanotube cathode, various devices can becreated, such as a field emitter array. An array can include a singlenanotube, a single bundle, or many carbon nanotubes and field emissiondisplay, e.g., a flat panel television. The treated carbon nanotube canconstitute the array. FIG. 10 is an illustration of a classical fieldemitter.

Table 3 shows example characteristics of a field emitter display TABLE 3emission type low & high voltage brightness (cd/m²) 150, 600 viewingangle (degrees) 160 emission efficiency (lm 10-15 response time 10-30contrast ratio >100:1 number of colors 16 million number of pixels640/480 resolution (mm pitch) 0.31 power consumption (W) 2 max screensize (cm) 26.4 panel thickness (mm) 10 operating temp range (° C.) −5 to85

The advantages of field emission display over other types of displayssuch as cathode ray tubes include: high brightness, peak brightness,full viewing angle, high emission efficiency, high dynamic range, fastresponse time and low power consumption.

Bibliography Use of Carbon Nanotubes in Field Emission Cathodes forLight Sources

PCT Appln. PCT/SE00/015221—A Light Source, and a Field Emission Cathode

Other Uses

PCT Appln. PCT/US99/13648—Free-Standing and Aligned Carbon Nanotubes andSynthesis Thereof (scanning electron microscope, alkali metal batteries,electromagnetic interference shield, and microelectrodes).

[Articles further describing the invention incorporated herein byreference:

-   Yahachi Saito et al., Cathode Ray Tube Lighting Elements with Carbon    Nanotube Field Emitters, 37 JAPAN. J. APPLIED PHYSICS 346 (1998).-   Yahachi Saito et al., Field Emissionfrom Multi-Walled Carbon    Nanotubes and its Application to Electron Tubes, 67 APPLIED PHYSICS    95, (1998).-   J. D. Carey et al., Origin of Electric Field Enhancement in Field    Emission from Amorphous Carbon Thin Films, 78 APPLIED PHYSICS    LETTERS 2339 (2001).-   Kenneth A. Dean et al., Current Saturation Mechanisms in Carbon    Nanotube Field Emitters, 76 APPLIED PHYSICS LETTERS 375 (2000).-   W. Zhu et al., Low-Field Electron Emission from Undoped    Nanostructured Diamond, 282 SCIENCE 1471 (1998).-   L. Nilsson et al., Carbon Nano-/Micro-Structures in Field Emission:    Environmental Stability and Field Enhancement Distribution, 383 THIN    SOLID FILMS 78 (2001).-   K. C. Walter et al., Improved Field Emission ofElectrons from Ion    Irradiated Carbon, 71 APPLIED PHYSICS LETTERS 1320 (1997)-   S. Dimitrijevic et al., Electron Emission From Films of Carbon    Nanotubes and ta-C Coated Nanotubes, 75 APPLIED PHYSICS LETTERS 2680    (1999)-   A. Wadhawan et al., Effects of Cs Deposition on the Field-Emission    Properties of Single-Walled Carbon-Nanotube Bundles, 78 APPLIED    PHYSICS LETTERS 108 (2001)-   O. Yavas et al., Improvement of Electron Emission ofsilicon Field    Emitter Arrays by Pulsed Laser Cleaning, 18 J. VAC. SCI. TECHNOL. B.    1081 (2000)-   O. Yavas, et al., Laser Cleaning of Field Emitter Arrays for    Enhanced Electron Emission, 72 APPLIED PHYSICS LETTERS 2797 (1998)-   M. Takai et al., Effect of Laser Irradiation on Electron Emission    from Si Field Emitter Arrays, 16 J. VAC. SCI. TECHNOL. B. 780 (1998)-   M. Takai et al., Electron Emission from Gated Silicide Field Emitter    Arrays, 16 J. VAC. SCI. TECHNOL. B. 790 (1998).]-   R. Khan et al. Electron Delocalization in Amorphous Carbon by Ion    Implantation, 63 PHYSICAL REVIEW B 121201-1 (2001)-   M. Takai et al., Effect of Gas Ambient on Improvement in Emission    Behavior ofSi Field Emitter Arrays, 16 J. VAC. SCI. TECHNOL. 799    (1998).-   O. Yavas et al., Field Emitter Array Fabricated Using Focused Ion    and Electron Beam Induced Reaction, 18 J. VAC. SCI. TECHNOL. 976    (2000)-   O. Yavas et al., Maskless Fabrication of Field-Emitter Array by    Focused Ion and Electron Beam, 76 APPLIED PHYSICS LETTERS 3319    (2000)-   A. Seidl et al., Geometry Effects Arising from Anodization of Field    Emitters, 18 J. VAC. SCI. TECHNOL. B 929 (2000).-   O. Yavas et al., Pulsed Laser Deposition of Diamond Like Carbon    Films on Gated Si Field Emitter Arrays for Improved Electron    Emission, 38 JAPAN. J. APPLIED PHYSICS 7208 (1999).

1. A field emission device comprising: a cathode; and an anode spacedfrom the cathode, wherein said cathode includes emitters comprisingcarbon nanotubes which have been subjected to energy, plasma, chemical,or mechanical treatment.
 2. The field emission device of claim 1,wherein said nanotubes are substantially cylindrical carbon fibrilshaving one or more graphitic layers concentric with their cylindricalaxes, said carbon fibrils being substantially free of pyrolyticallydeposited carbon overcoat, having a substantially uniform diameterbetween 1 nm and 100 nm and having a length to diameter ratio greaterthan
 5. 3. The field emission device of claim 1, wherein said nanotubesare in the form of aggregates selected from the group consisting ofcotton candy aggregates.
 4. The field emission device of claim 1,wherein said nanotubes have a morphology resembling a fishbone.
 5. Thefield emission device of claim 1, wherein said nanotubes are single wallnanotubes.
 6. The field emission device of claim 1, wherein saidnanotubes are in the form of a film or mat.
 7. The field emission deviceof claim 1, wherein said nanotubes have been treated with an ion beam.8. The field emission device of claim 1, wherein said nanotubes havebeen treated with a gallium ion beam.
 9. The field emission device ofclaim 1, wherein said nanotubes have been treated with a beam of ionsselected from the group consisting of hydrogen, helium, argon, carbon,oxygen, and xenon ions.
 10. The field emission device of claim 1,wherein said chemical treatment is selected from the group consisting ofacid treatment, metal vapor treatment, chemical vapor transport andchemical sorption.
 11. The field emission device of claim 1, whereinsaid chemical treatment is performed with chemical reagents selectedfrom the group consisting of oxidizing agents, electrophiles,nucleophiles, reducing agents, strong acids, strong bases and mixturesthereof.
 12. The field emission device of claim 1, wherein said chemicaltreatment is performed with phthalocyanines or porphyrins.
 13. The fieldemission device of claim 1 wherein said energy treatment is performedwith an energy source selected from a group consisting ofelectromagnetic radiation, ionizing radiation, atomic beams, electronbeams, ultraviolet light, microwave radiation, gamma ray, x-ray, neutronbeam, molecular beams and laser beam.
 14. The field emission device ofclaim 1 wherein said plasma treatment is performed with a plasmaselected from a group consisting of oxygen, hydrogen, ammonia, helium,argon, water, nitrogen, ethylene, carbon tetrafluoride, sulfurhexafluoride, perfluoroethylene, fluoroform, difluoro-dichloromethane,bromo-trifluoromethane, chlorotrifluoromethane and mixtures thereof. 15.The field emission device of claim 1, wherein said treatment results inthe introduction of metal atoms onto the carbon nanotubes.
 16. The fieldemission device of claim 1, wherein said treatment results in theintroduction of functional groups onto the carbon nanotubes.
 17. Thefield emission device of claim 16, wherein said functional groups havebeen introduced by chemical sorption.
 18. The field emission device ofclaim 1, wherein said treatment comprises heating the carbon nanotubesin the presence of metal vapor.
 19. The field emission device of claim1, wherein said treatment comprises chemisorption followed by heattreatment.
 20. The field emission device of claim 1, wherein saidtreatment includes annealing said nanotubes.
 21. The field emissiondevice of claim 1, wherein said cathode further includes a binder. 22.The field emission device of claim 21, wherein said binder is aconductive carbon paste, conductive metal paste or carbonizable polymer.23. A process for treating carbon nanotubes comprising the step ofbombarding carbon nanotubes with ions.
 24. The process for treatingcarbon nanotubes of claim 23, wherein the nanotubes are bombarded withgallium ions.
 25. The process of treating carbon nanotubes of claim 23,wherein the nanotubes are bombarded with ions selected from the groupconsisting of hydrogen, helium, argon, carbon, oxygen, and xenon ions.26. Carbon nanotubes formed by the process of claim
 23. 27. A fieldemission cathode comprising carbon nanotubes, wherein said nanotubeshave been subjected to energy, plasma, chemical, or mechanicaltreatment.
 28. The field emission cathode of claim 27, wherein saidnanotubes are substantially cylindrical carbon fibrils having one ormore graphitic layers concentric with their cylindrical axes, saidcarbon fibrils being substantially free of pyrolytically depositedcarbon overcoat, having a substantially uniform diameter between 1 nmand 100 nm and having a length to diameter ratio greater than
 5. 29. Thefield emission cathode of claim 27, wherein said nanotubes are in theform of aggregates selected from the group consisting of cotton candyaggregates or bird nest aggregates.
 30. The field emission cathode ofclaim 27, wherein said nanotubes have a morphology resembling afishbone.
 31. The field emission cathode of claim 27 wherein saidnanotubes are single wall nanotubes.
 32. The field emission cathode ofclaim 27, wherein said nanotubes are in the form of a film or mat. 33.The field emission cathode of claim 27, wherein said nanotubes have beentreated with an ion beam.
 34. The field emission cathode of claim 27,wherein said nanotubes have been treated with a gallium ion beam. 35.The field emission cathode of claim 27, wherein said nanotubes have beentreated with a beam of ions selected from the group consisting ofhydrogen, helium, argon, carbon, oxygen, and xenon ions.
 36. The fieldemission cathode of claim 27, wherein said chemical treatment isselected from the group consisting of acid treatment, metal vaportreatment, chemical vapor transport, and chemical sorption.
 37. Thefield emission cathode of claim 27, wherein said chemical treatment isperformed with chemical reagents selected from the group consisting ofoxidizing agents, electrophiles, nucleophiles, reducing agents, strongacids, strong bases and mixtures thereof.
 38. The field emission cathodeof claim 27, wherein said chemical treatment is performed withphthalocyanines or porphyrins.
 39. The field emission cathode of claim27, wherein said energy treatment is performed with an energy sourceselected from a group consisting of electromagnetic radiation, ionizingradiation, atomic beams, electron beams, ultraviolet light, microwaveradiation, gamma ray, x-ray, neutron beam, molecular beams and laserbeam.
 40. The field emission cathode of claim 27, wherein said plasmatreatment is performed with a plasma selected from a group consisting ofoxygen, hydrogen, ammonia, helium, argon, water, nitrogen, ethylene,carbon tetrafluoride, sulfur hexafluoride, perfluoroethylene,fluoroform, difluoro-dichloromethane, bromo-trifluoromethane,chlorotrifluoromethane and mixtures thereof.
 41. The field emissioncathode of claim 27, wherein said treatment results in the introductionof metal atoms onto the carbon nanotubes.
 42. The field emission cathodeof claim 27, wherein said treatment results in the introduction offunctional groups onto the carbon nanotubes.
 43. The field emissioncathode of claim 42, wherein said functional groups have been introducedby chemical sorption.
 44. The field emission cathode of claim 27,wherein said treatment comprises heating the carbon nanotubes in thepresence of metal vapor.
 45. The field emission cathode of claim 27,wherein said treatment comprises chemisorption followed by heattreatment.
 46. The field emission cathode of claim 27, wherein saidtreatment includes annealing said nanotubes.
 47. The field emissioncathode of claim 27, wherein said cathode further includes a binder. 48.The field emission cathode of claim 47, wherein said binder is aconductive carbon paste, a conductive metal paste or a carbonizablepolymer.
 49. The field emission cathode of claim 27, wherein thenanotubes are deposited onto a substrate.
 50. A method for making afield emission cathode comprising the steps of: dispersing carbonnanotubes into a liquid vehicle to form a solution; forming anelectrophoresis bath, said bath including an anode and a cathodeimmersed therein; applying a voltage to said anode and said cathode,thereby causing said carbon nanotubes to deposit onto said cathode;removing said cathode from said bath; and subjecting the nanotubesdeposited on said cathode to an energy, plasma, chemical, or mechanicaltreatment.
 51. The method for making a field emission cathode of claim50, wherein said nanotubes are substantially cylindrical carbon fibrilshaving one or more graphitic layer concentric with their cylindricalaxes, said carbon fibrils being substantially free of pyrolyticallydeposited carbon overcoat, having a substantially uniform diameterbetween 1 nm and 100 nm and having a length to diameter ratio greaterthan
 5. 52. The method for making a field emission cathode of claim 50,wherein said nanotubes are in the form of aggregates selected from thegroup consisting of cotton candy aggregates or bird nest aggregates. 53.The field emission display device of claim 50, wherein said nanotubeshave a morphology resembling a fishbone.
 54. The method of making afield emission cathode of claim 50, wherein said nanotubes are singlewall nanotubes.
 55. The method of making a field emission cathode ofclaim 50, wherein said nanotubes are in the form of a film or mat. 56.The method for making a field emission cathode of claim 50, wherein saidcathode is bombarded with ions.
 57. The method for making a fieldemission cathode of claim 50, wherein said cathode is bombarded with agallium ions.
 58. The method for making a field emission cathode ofclaim 50, wherein said cathode is bombarded with ions selected from thegroup consisting of hydrogen, helium, argon, carbon, oxygen, and xenonions.
 59. The method for making a field emission cathode of claim 50,wherein said chemical treatment is selected from the group consisting ofacid treatment, metal vapor treatment, chemical vapor transport, andchemical sorption.
 60. The method for making a field emission cathode ofclaim 50, wherein said chemical treatment is performed with chemicalreagents selected from the group consisting of oxidizing agents,electrophiles, nucleophiles, reducing agents, strong acids, strong basesand mixtures thereof.
 61. The method for making a field emission cathodeof claim 50, wherein said chemical treatment is performed withphthalocyanines or porphyrins.
 62. The method for making a fieldemission cathode of claim 50, wherein said energy treatment is performedwith an energy source selected from a group consisting ofelectromagnetic radiation, ionizing radiation, atomic beams, electronbeams, ultraviolet light, microwave radiation, gamma ray, x-ray, neutronbeam, molecular beams and laser beam.
 63. The method for making a fieldemission cathode of claim 50, wherein said plasma treatment is performedwith a plasma selected from a group consisting of oxygen, hydrogen,ammonia, helium, argon, water, nitrogen, ethylene, carbon tetrafluoride,sulfur hexafluoride, perfluoroethylene, fluoroform,difluoro-dichloromethane, bromo-trifluoromethane, chlorotrifluoromethaneand mixtures thereof.
 64. The method for making a field emission cathodeof claim 50, wherein said treatment results in the introduction of metalatoms onto the carbon nanotubes.
 65. The method for making a fieldemission cathode of claim 50, wherein said treatment results in theintroduction of functional groups onto the carbon nanotubes.
 66. Themethod for making a field emission cathode of claim 65, wherein saidfunctional groups have been introduced by chemical sorption.
 67. Themethod for making a field emission cathode of claim 50, wherein saidtreatment comprises heating said cathode in the presence of metal vapor.68. The method for making a field emission cathode of claim 50, whereinsaid treatment comprises chemisorption followed by heat treatment. 69.The method for making a field emission cathode of claim 50, wherein saidtreatment includes annealing said nanotubes.
 70. The method for making afield emission cathode of claim 50, further comprising the step ofadding a binder to said solution before applying said voltage.
 71. Themethod for making a field emission cathode of claim 70, wherein saidbinder is a conductive carbon paste, a conductive metal paste or acarbonizable polymer.
 72. A field emission display device comprising: acathode including carbon nanotubes which have been subjected to energy,plasma, chemical, or mechanical treatment; an insulating layer on saidcathode; a gate electrode on said insulating layer; an anode spaced fromsaid cathode, said anode comprising a phosphor layer, an anodeconducting layer, and a transparent insulating substrate; and a powersupply.
 73. A method for making a field emission cathode comprising thesteps of: screen printing an ink onto a substrate, said ink comprising acarrier liquid and carbon nanotubes in as-made form or which have beensubjected to energy, plasma, chemical, or mechanical treatment; andevaporating said carrier liquid.
 74. A method for making a fieldemission cathode comprising the steps of: ink jet printing an ink onto asubstrate, said ink comprising a carrier liquid and carbon nanotubeswhich have been in as-made form or which have been subjected to energy,plasma, chemical, or mechanical treatment; and evaporating said carrierliquid.
 75. A method for making a field emission cathode comprising thesteps of: spray painting an ink through a stencil onto a substrate, saidink comprising a carrier liquid and carbon nanotubes which have beensubjected to energy, plasma, chemical, or mechanical treatment; andevaporating said carrier liquid.
 76. A method for making a fieldemission cathode comprising the steps of: screen printing, ink-jetprinting or spray painting an ink onto a substrate, said ink comprisinga carrier liquid and carbon nanotubes in as-made form; and subjectingsaid screen printed nanotubes to an energy, plasma, chemical ormechanical treatment.
 77. A field emission cathode made by the method ofclaim
 107. 78. A field emission display device comprising: a baseplate;an electron emitter array, said array including carbon nanotubes whichhave been subjected to energy, plasma, chemical, or mechanicaltreatment; a gate on said baseplate; a phosphor coated faceplate spacedfrom said gate; a faceplate on said phosphor coated faceplate; and apower supply.
 79. A field emission device comprising: a substrate, aporous top layer on said substrate, a catalyst material on said layer;and a cathode on said catalyst material, said cathode including a bundleof carbon nanotubes which have been subjected to energy, plasma,chemical, or mechanical treatment.